1. Field of the Invention
This invention relates to preventing audio pop in an electronic audio device. More particularly, the invention relates to preventing audio pop in a digital audio circuit, such as in a cellular telephone.
2. Background of Related Art
An electronic device with an audio output, such as for example a cellular telephone, may include one or more audio channels. FIG. 5 shows a conventional audio output channel in a digital electronic audio device.
In particular, as shown in FIG. 5, a circuit such as that found in conventional cellular telephones includes an audio output channel driven by a digital audio source (e.g., a digital signal processor (DSP)) 100. The digital output signal corresponding to the relevant audio is output from the DSP 100 and converted to an analog signal by a digital-to-analog (D/A) converter 110 (e.g., a COder/DECoder (codec)), followed by an analog filter 120, and an amplifier 130. A speaker or headset load 150 is AC coupled to the output of the amplifier 130, represented in FIG. 5 by a capacitor 140.
However, when such a conventional audio output channel is powered on, a DC component of the output voltage of the amplifier 130 suddenly increases from, e.g., zero volts to it's DC working level of voltage. This sudden voltage change, when coupled through the AC coupling capacitor 140, causes the AC coupling capacitor to be charged rapidly, thus having a tendency to cause an annoying popping sound at the audio load 150 (e.g., speaker or headset), particularly if powered on at the peak of a loud waveform. Similarly, even when the audio circuit is turned off and the supply voltage is removed from the audio amplifier stage 130, the DC output voltage level suddenly decreases, rapidly discharging the AC coupling capacitor 140, thus causing a further popping sound to the listener through the audio load 150. In extreme cases, the cumulative effect of such loud popping sounds may even be harmful to the hearing of the listener.
Carroll et al. show in U.S. Pat. No. 6,157,726 a previous attempt to solve the above problem, reproduced in FIG. 6 of this application.
In particular, as shown in FIG. 6 herein, the output of the audio amplifier 130 is AC coupled to a parallel combination of two paths, each including a switch 570, 580. One path includes the audio load 150 and the switch 580, while the other path includes a dummy load 560 and the other switch 570. The audio load 150 may be a speaker, headset, or the like, whereas the dummy load 560 may be simply a resistor or other resistive device.
In the operation of the circuit of FIG. 6, the first analog switch 570 is closed and the second analog switch 580 is open when the supply voltage is initially applied to amplifier 530. The rising DC output voltage from amplifier 530 therefore charges the AC coupling capacitor 540, but the capacitor 540 is connected to the dummy load 560, and so no popping sound is heard at the load 550. Then, after the AC coupling capacitor 540 has been charged up, the first analog switch 570 is opened and the second analog switch 580 is closed, connecting the amplifier 530 to the load 550 via the AC coupling capacitor 540. Before the supply voltage to the amplifier 530 is turned off, the second analog switch 580 is re-opened and first analog switch 570 is re-closed, so that the AC coupling capacitor is discharged through the dummy load 560, thus avoiding a popping sound heard by the listener at the actual audio load 550.
However, the circuit of FIG. 6 is somewhat disadvantageous in that it requires not only additional components in the audio path itself (e.g. two switches 570, 580 and a dummy load 560), but also some form of control circuitry, including circuitry needed to sense when the audio output channel is being turned on or off.
There is a need for a technique and apparatus to eliminate a popping sound without the introduction of additional and/or complex circuitry.